Shrink labels represent a high growth area for labels, particularly for high shrink sleeve labels. There is a growing trend toward use of contoured containers with shrink sleeve labels having unique 360° graphics to provide a trademark package with high customer appeal.
Current high shrink films include those made from polyvinylchloride (PVC), polyethylene teraphthalate glycol (PETG) or co-polyesters, and oriented polystyrene (OPS). All of these provide over 50% TD (transverse direction) shrinkage and are solvent-sealable, ideal qualities for use as sleeve labels. Although they provide very good shrink properties, all of these films have some deficiencies. Recyclability is a major issue, especially with the growth of PET bottles. PVC and PETG have densities similar to PET, and therefore cannot be separated from the bottle material during the recycle process. PVC will contaminate the PET, and also suffers from a negative environmental perception. The density of OPS is different enough from PET for separation, but since its density is slightly greater than 1.0 g/cm3, it won't float in a standard water separation recycle stream. Storage stability, or lack thereof, is another area for improvement of currently available shrink sleeve films. Due to the low shrink onset temperatures of these materials, the moderately high temperatures seen during transport and storage will result in some shrinkage and therefore poor roll quality. These films must be kept refrigerated during transport and storage and have a recommended usable life of less than 6 months. Additionally, film cost is high, especially for PETG/co-polyester films.
Oriented polypropylene (OPP) films can be used in shrink labels, although they are best suited for lower shrink (i.e. <25% shrink) applications. These are typically MD (machine direction) shrink films for roll-fed label applications, using a hot melt adhesive for the seam. Current OPP shrink films are limited by the overall amount of shrink, high temperatures needed for shrinking, and hot melt seaming technology. Otherwise, OPP films do have benefits in cost, recyclability, and storage temperature stability.
There have been some disclosures of OPP-containing shrink films. Bioriented films produced using a tenter process for polyethylene based films is described in U.S. Pat. No. 2,241,030. Shrink films comprising a blend of a polypropylene with ethylene-based polymers are disclosed in U.S. Pat. No. 6,908,687. An oriented film comprising metallocene-produced polypropylene is disclosed in U.S. Pat. No. 6,242,084, wherein at least one layer is “voided” to increase the oxygen and water vapor transmission. Voiding may be useful for food wrap films, but not necessarily for shrink wrap films for labels. A polypropylene containing core layer is described in shrink films disclosed in U.S. Pat. No. 6,025,079, but the outer (“skin”) layers are not solvent sealable. A metallocene produced propylene copolymer for use in a heat-sealable skin layer of a film is disclosed in U.S. Pat. No. 7,351,478. None of these disclosures fully solves the problem of providing a solvent-sealable, recyclable oriented film for shrink sleeve labels.
It is therefore desirable to produce a high shrink film that is solvent sealable, thermally stable during transport/storage, cost effective, and has density less than 1.0 g/cm3 for recyclability.